Use of calcuim carbonate particles in papermaking

ABSTRACT

A process for the production of paper by introducing a suspension of ultrafine calcium carbonate particles to a fiber-containing suspension. A method for improving the dewatering of a fiber-containing suspension. Use of a calcium carbonate suspension comprising ultrafine particles of calcium carbonate as a substitute for colloidal silica as a dewatering agent in the formation of paper from a fiber-containing suspension. At least one ionic polymer is introduced in the fiber-containing suspension.

The present invention relates to the use of a calcium carbonate (CC) in papermaking. More specifically, it relates to the use of CC to improve the dewatering and retention properties of paper in papermaking. The invention also leads to paper with improved optical properties and improved printability.

It is well known to use colloidal silica, such as that associated with the various Compozil® systems, in the production of paper to obtain improved retention and dewatering. However colloidal silica is an expensive material and there is a need for an alternative cheaper material.

It has been surprisingly discovered that colloidal silica can be replaced by ultrafine particles of calcium carbonate. Here, a dry calcium carbonate or a suspension of CC, especially a suspension comprising ultrafine precipitated calcium carbonate (PCC) particles is utilized in the manner analogous to the previously employed colloidal silica. These suspensions can provide a substantial improvement of the retention and dewatering in the production of paper. The improvements in dewatering can, for example, allow the speed of the papermaking machine to be increased. Accordingly, the economics of the papermaking process can be substantially improved. Furthermore, the obtained paper shows improved optical properties and an improved printability.

The invention then relates to a process for the production of paper, comprising the steps of:

-   (a) providing a fiber-containing suspension containing cellulose     fibers, and optional fillers; -   (b) introducing at least one suspension comprising ultrafine     particles of calcium carbonate to the fiber-containing suspension,     and -   (c) forming and dewatering the fiber-containing suspension resulting     from step (b)     wherein the ultrafine calcium carbonate particles have an average     size lower than or equal to 200 nm.

The invention also relates to a method for improving the dewatering of a fiber-containing suspension containing cellulose fibers comprising the addition of a calcium carbonate suspension comprising ultrafine particles of calcium carbonate to a fiber-containing suspension containing cellulose fibers, wherein the ultrafine calcium carbonate particles have an average size lower than or equal to 200 nm.

The invention also relates to the use of a calcium carbonate suspension comprising ultrafine particles of calcium carbonate as a substitute for colloidal silica as a dewatering agent in the formation of paper from a fiber-containing suspension containing cellulose fibers, wherein the ultrafine calcium carbonate particles have an average size lower than or equal to 200 nm.

The particles of calcium carbonate according to the invention can be particles of natural or synthetic calcium carbonate.

Natural calcium carbonate can be processed by mechanically crushing and grading calcareous ore to obtain particles adjusted to the desired size and surface area. Synthetic calcium carbonate is usually prepared by precipitation using various sources of calcium and carbonates ions. Precipitated calcium carbonate (PCC) is preferred.

Particles of PCC may be manufactured by first preparing a calcium oxide (quick lime) by subjecting limestone to calcination by burning a fuel, such as coke, a petroleum fuel (such as heavy or light oil), natural gas, petroleum gas (LPG) or the like, and then reacting the calcium oxide with water to produce a calcium hydroxide slurry (milk of lime), and reacting the calcium hydroxide slurry with carbon dioxide to obtain the desired particle size and shape PCC (carbonation process). Carbon dioxide can be discharged from a calcination furnace for obtaining the calcium oxide from limestone, from gases from power plants or from liquid CO₂ containers for instance. It is preferred to use carbon dioxide discharged from a calcination furnace for obtaining the calcium oxide from limestone. Precipitation of calcium carbonate can also be carried out by adding an alkali metal carbonate starting with lime water (caustification method) or by the addition of an alkali metal carbonate starting with solutions containing calcium chloride. PCC particles obtained from the carbonation process are preferred.

The calcium carbonate can be substantially amorphous or substantially crystalline. Substantially amorphous or crystalline is understood to mean that more than 50% by weight of the calcium carbonate is in the form of amorphous or crystalline material when analyzed by an X-ray diffraction technique. Substantially crystalline calcium carbonate is preferred. Crystalline calcium carbonate can consist of calcite or aragonite or a mixture of these two crystalline phases. The calcite phase is preferred.

In the case where the calcium carbonate is synthetic calcium carbonate, the particles can be of any shape. They may have the form of needles, scalenohedrons, rhombohedrons, spheres, platelets or prisms. A rhombohedral shape, that can be reduced to pseudo-cubes or pseudo-spheres, is preferred.

The calcium carbonate particles according to the invention have usually a BET specific surface area higher than or equal to 10 m²/g, preferably higher than or equal to 20 m²/g, more preferably higher than or equal to 40 m²/g, still more preferably higher than or equal to 60 m²/g and in particular higher than or equal to 70 m²/g. The particles according to the invention have generally a BET specific surface area lower than or equal to 300 m²/g preferably lower than or equal to 250 m²/g, more preferably lower than or equal to 200 m²/g, still more preferably lower than or equal to 150 m²/g and in particular lower than or equal to 100 m²/g. The skilled person is aware how to determine the BET specific surface area of the particles. Preferably, the BET specific surface area is measured according to the ISO 9277 norm.

The “ultrafine” calcium carbonate particles according to the invention have usually a mean elementary particle size (dp) higher than or equal to 5 nm, preferably higher than or equal to 10 nm, more preferably higher than or equal to 10 nm, still more preferably higher than or equal to 15 nm and most preferably higher than or equal to 20 nm. The mean particle size is generally lower than or equal to 200 nm, preferably lower than or equal to 150 nm, more preferably lower than or equal to 100 nm and most preferably lower than or equal to 70 nm. The skilled person is aware of suitable methods for determining the mean elementary particle size. In this regard it can be referred e.g. to norm NF 11 601/11 602.

The calcium carbonate particles according to the invention can be coated with at least one coating agent. The coating agent is selected from carboxylic acids, carboxylic acids salts, sulfonic acids, sulfonic acid salts, alkylsulfates, alkylsulfosuccinates and mixtures thereof, to mention only a few.

It is preferred to use uncoated calcium carbonate.

For example, the PCC suspension according to the present invention can comprise ultrafine PCC particles such as those available from Solvay SA under the SOCAL® trademark. Specific examples of such ultrafine particles are illustrated in Table 1. In view of their size, the ultrafine particles employed in the present invention typically have a high surface area and a high charge density which make them particularly suitable for use in the invention.

Although not wishing to be bound by a particle theory, it is believed that the primary function of the CC particles is the ability to provide a desired charge, e.g., typically a negative charge, thereby enhancing the charge characteristics of the system.

The ultrafine particles can be employed as a dry solid (powder) or in an organic or aqueous suspension in an amount suitable to provide the desired dewatering improvement. By dry solid, one intends to denote a solid for which the water content can be less than or equal to 10% by weight. This content is preferably less than or equal to 3% by weight and more particularly less than or equal to 1% by weight. It is preferred to use aqueous suspensions of ultrafine particles of CC. It is most preferred to use aqueous suspensions of ultrafine particles of precipitated calcium carbonate. Those suspensions can be made by dispersing dry precipitated calcium carbonate into water or can be calcium carbonate suspensions resulting from the precipitation processes.

In this regard, it may be desirable to provide as high as concentration of the particles as possible subject to issues such as the manufacturing conditions, the maximum concentration at which the suspension would remain fluid and pourable without excessive settling. The content of ultrafine particles in the suspension of calcium carbonate is usually higher than or equal to 1% by weight, preferably higher than or equal to 5% by weight and most preferably higher than or equal to 8% by weight. The content of ultrafine particles in the suspension is usually lower than or equal to 70% by weight, preferably lower than or equal to 50% by weight, more preferably lower than or equal to 40% by weight and most preferably lower than or equal to 20% by weight. One example of a suitable suspension includes about 10% by weight of the ultrafine particles in an aqueous suspension.

As mentioned above, the suspensions according to the present invention can be used as a replacement for colloidal silica in paper-making processes. Insofar as paper-making processes are well recognized in the art, they need not be described in detail. However, for sake of completeness, the inventors offer the following remarks regarding the paper-making suspensions that are suitable for the present invention.

The present invention can employ a variety of paper-making suspensions containing a variety of cellulose-containing fibers. The suspensions should typically contain a suitable amount of fibers to provide the desired consistency at the various points of the paper making process. For example, the consistency of the fiber in thick stock can typically be on the order of 3%, of thin stock on the order of 0.5 to 1% and later, at the drying section, at least about 50 percent by weight of such fibers, based on dry material. Such amounts are well recognized in this field.

The components can for example be used for suspensions of fibers from chemical pulp, such as sulphate and sulphite pulp, thermomechanical pulp, refiner pulp or groundwood pulp from both hardwood and softwood and can also be used for suspensions based on recycled fibers. The suspension can also contain mineral fillers, such as for example kaolin, titanium dioxide, gypsum, chalk and talcum.

Finally, it is noted that the terms “paper” and “paper-making” as used herein do of course not include solely paper and its production but also other cellulose fiber containing products in sheet or web form such as pulp sheets, board and cardboard and their production.

The amount of calcium carbonate in the fiber containing suspension is to a high degree dependent on the type effects desired from this. It is generally higher than or equal to 0.01% by weight calculated as dry calcium carbonate on the sum of dry fibers and optional fillers, preferably higher than or equal to 0.05 wt %, more preferably higher than or equal to 0.1 wt % and most preferably higher than or equal to 0.15 wt %. This amount is usually lower than or equal to 50 wt %, preferably lower than or equal to 40 wt %, more preferably lower than or equal to 30 wt % and most preferably lower than or equal to 20 wt %.

The calcium carbonate suspensions in the process, method and use according to the invention can be used in combination with agents employed in the paper-making suspension. Among such agents, ionic polymers are preferred.

The invention then also relates to process, a method and a use as described above, where at least one ionic polymer is introduced in the fiber-containing suspension containing cellulose fibers. Among ionic polymers, cationic polymers are preferred. The cationic polymers suitable for use in the invention include natural, e.g. based on carbohydrates, and synthetic polymers. Examples of suitable polymers include cationic starch, cationic guar gum, cationic acrylamide based polymers, cationic polyethyleneimines, polyamidoamines and poly(diallyldimethyl ammonium chloride). The polymers can be used singly or in combination with each other. Cationic starch is preferred and can be selected from starch tertiary aminoalkyl ethers derivatives, starch quaternary ammonium ethers derivatives, aminoethylated starches, starch cyanamide derivatives, starch anthranilates and cationic dialdehyde starch. These cationic starches are produced by well-know reactions from starches arising from various sources like corn, potatoes, wheat and rice, for instance.

The amount of polymer is to a high degree dependent on the type of this and other effects desired from this. For synthetic polymers at least 0.01 kg polymer per ton, calculated as dry on the sum of dry fibers and optional fillers are usually used. Suitably amounts of from 0.01 to 3 and preferably from 0.03 to 2 kg per ton are used. For polymers based on carbohydrates, such as cationic starch and cationic guar gum, typically, amounts of at least 0.1 kg/ton, calculated as dry on the sum of dry fibers and optional fillers, are used. Suitably these are used in amounts of from 0.5 to 30 kg/ton and preferably from 1 to 15 kg/ton.

The other significant component of the paper-making suspension is the CC suspension. The amount of CC suspension employed in the context of the present invention can vary within wide limits depending on, among other things, the type of suspension being employed.

In the context of the present invention, the weight ratio of cationic polymer(s) to CC is typically based on the charge characteristics of the system. The other primary factor relates to the economics of the system. It is particularly suitable where the ratio of polymer to CC in suspension is not less than 0.10 wt/wt, more preferably not less than 0.20 wt/wt.

However, it is important to note that a wide range of amounts for the CC suspension employed is capable of providing the dewatering advantages that can be associated with the present invention.

The paper-making suspensions employed in the present invention can include one or more conventional paper additives such as hydrophobing agents, dry strength agents, wet strength agents etc. Such additives are suitable for, but not significant to, the present invention.

Examples of suitable additive include aluminum compounds that can be employed in combination with the CC suspension and cationic polymers, since it has been found that aluminum compounds may provide a further improvement of retention and dewatering. Any known aluminum compound for use in papermaking can be used, for example alum, polyaluminium compounds, aluminates, aluminum chloride and aluminum nitrate. The polyaluminium compounds can for example be polyaluminium chlorides, polyaluminium sulphates and polyaluminium compounds containing both chloride and sulphate ions. The polyaluminium compounds can also contain other anions than chloride ions, for example anions from sulphuric acid, phosphoric acid, organic acids such as citric acid and oxalic acid.

In addition, while the cationic polymer(s) are typically added prior to the CC suspension, processes employing a reversed order of addition are not outside the scope of this invention.

Upon adding the CC suspension to the fiber-containing suspension, the process includes the forming and dewatering of the fiber-containing suspension on a wire to form paper. In this regard, techniques and devices for forming and dewatering the paper-making suspension are well-recognized in the art and need not be described in detail here. It is noted that the CC suspension can be effectively employed over the entire pH range of 4 to 10 in papermaking, with 4.5 to 8.5 being typically preferred.

The invention is further illustrated in the following example which, however, is not intended to limit the same.

EXAMPLE Comparison of Ultrafine PCC Suspension to Compozil® Components

Colloidal silica: Compozil® BMA 0, anionic silica sol used directly as sold, 10% by weight suspension.

Calcium carbonate: SOCALO® U3: A, 10% by weight suspension of SOCAL® U3 (SOLVAY).

(Stirring Speed 1500 rpm, Small Toothed-disk Stirrer)

Cationic potatoe Starch with 0.4-0.5% of N: a 1% by weight hot suspension of starch.

Production of the Paper Material:

The fiber containing suspension has been diluted to a concentration of approximately 7.0 g/l by addition of deionized water. The concentration has been adjusted by testing the total retention (Procedure P.5.). Then NaCl was added to attain a conductance of 1.2 mS/cm. This stock has then been divided into 1000-ml portions.

The fiber concentrations were 6.51 g/l for samples employing calcium carbonate et 6.84 g/l for samples employing colloidal silica.

The required amounts of starch paste and/or calcium carbonate suspension or colloidal silica suspensions have then been added to these portions, and the mixtures have been subjected to defined shearing using a Dynamic Filtration System DFS 03 (Overseas instruments), and then desiccated.

Determination of the Drainage Time

1000 ml of the paper-stock/starch mixture were emptied through the funnel of the DFS03 into the stirring chamber (which is intended for the desiccation time) and then stirred at 500 rpm. The stirrer stopped after 60 s, and at the same time the sealing cone of the stirring chamber was lifted. The amount of filtrate thus produced was measured vs. time. The maximum amount of filtrate was 400 g. The filtration was done through a Schopper-Riegler standard metal screen.

Table 2 illustrates that the results for calcium carbonate while, Table 3 shows the results for colloidal silica in tabular form.

TABLE 2 Effect of calcium carbonate on the Drainage Time CC susp Starch Starch/dry CC/dry Starch/ 10% wt. 1% wt fibers fibers CC Drainage (ml) (ml) (wt %) (wt %) (wt/wt) Time (sec) Without 0 0 0.0 0.0 0 148 CC 0 10 1.5 0.0 0 181 Without 2 0 0.0 3.0 0 149 Starch 5 0 0.0 7.6 0 180 10 0 0.0 15.2 0 200 Starch 10 10 1.5 15.2 0.1 97 and CC 5 10 1.5 7.6 0.2 75 2 10 1.5 3.0 0.5 73 1 10 1.5 1.5 1.0 63 0.1 10 1.5 0.15 10 55

TABLE 3 Effect of colloidal silica on the Drainage Time Compozil Starch Starch/dry Compozil/ Starch/ susp 10% wt. 1% wt fibers dry fibers Composil Drainage (ml) (ml) (wt %) (wt %) (wt/wt) time (sec) Without 0 0 0.0 0.0 0 169 Compozil 0 10 1.5 0.0 0 156 0 10 1.5 0.0 0 205 Without 2 0 0.0 2.9 0 173 Starch 5 0 0.0 7.3 0 178 10 0 0.0 14.6 0 136 20 0 29.2 0 193 Starch and 20 10 1.5 29.2 0.05 190 Compozil 10 10 1.5 14.6 0.1 210 5 10 1.5 7.3 0.2 187 2 10 15 2.9 0.5 162 1 10 1.5 1.5 1.0 127 0.5 10 1.5 0.75 2.0 67 0.2 10 1.5 0.3 5.0 52 0.1 10 1.5 0.15 10 55 

1-10. (canceled)
 11. A process for the production of paper, comprising the steps of: (a) providing a fiber-containing suspension containing cellulose fibers, and optional fillers; (b) introducing at least one suspension comprising ultrafine particles of calcium carbonate to the fiber-containing suspension, and (c) forming and dewatering the fiber-containing suspension resulting from step (b), wherein the ultrafine calcium carbonate particles have an average size lower than or equal to 200 nm.
 12. A method for improving the dewatering of a fiber-containing suspension containing cellulose-fibers, said method comprising the addition of a calcium carbonate suspension comprising ultrafine particles of calcium carbonate to a fiber-containing suspension containing cellulose-fibers, wherein the ultrafine calcium carbonate particles have an average size lower than or equal to 200 nm.
 13. A method which comprises the use of a calcium carbonate suspension comprising ultrafine particles of calcium carbonate as a substitute for colloidal silica as a dewatering agent in the formation of paper from a fiber-containing suspension containing cellulose-fibers, wherein the ultrafine calcium carbonate particles have an average size lower than or equal to 200 nm.
 14. The process according to claim 11 wherein at least one ionic polymer is introduced into the fiber-containing suspension containing cellulose fibers.
 15. The process according to claim 11 wherein the calcium carbonate is a precipitated calcium carbonate and wherein the calcium carbonate is calcite.
 16. The process according to claim 11 wherein the content of ultrafine particles of calcium carbonate in the calcium carbonate suspension is higher than or equal to 1% by weight and lower than or equal to 70% by weight.
 17. The process according to claim 11 wherein the ratio of the cationic polymer with respect to the calcium carbonate is higher than or equal to 0.1 wt/wt.
 18. The process according to claim 11 wherein the amount of calcium carbonate in the fiber-containing suspension containing cellulose fibers is higher than or equal to 0.01% and lower than or equal to 50% by weight calculated as dry calcium carbonate on the sum of dry fibers and optional fillers.
 19. The process according to claim 11 wherein the ionic polymer is a cationic polymer selected from the group consisting of cationic starch, cationic guar gum, cationic acrylamide based polymers, cationic polyethyleneimines, polyamidoamines and poly(diallyldimethyl ammonium chloride) or any mixture thereof.
 20. The process according to claim 19 wherein said cationic starch is selected from the group consisting of starch tertiary aminoalkyl ether derivatives, starch quaternary ammonium ether derivatives, aminoethylated starches, starch cyanamide derivatives, starch anthranilates and cationic dialdehyde starch.
 21. The method according to claim 12 wherein at least one ionic polymer is introduced into the fiber-containing suspension containing cellulose fibers.
 22. The method according to claim 12 wherein the calcium carbonate is a precipitated calcium carbonate and wherein the calcium carbonate is calcite.
 23. The method according to claim 12 wherein the content of ultrafine particles of calcium carbonate in the calcium carbonate suspension is higher than or equal to 1% by weight and lower than or equal to 70% by weight.
 24. The method according to claim 12 wherein the ratio of the cationic polymer with respect to the calcium carbonate is higher than or equal to 0.1 wt/wt.
 25. The method according to claim 12 wherein the amount of calcium carbonate in the fiber-containing suspension containing cellulose fibers is higher than or equal to 0.01% and lower than or equal to 50% by weight calculated as dry calcium carbonate on the sum of dry fibers and optional fillers.
 26. The method according to claim 12 wherein the ionic polymer is a cationic polymer selected from the group consisting of cationic starch, cationic guar gum, cationic acrylamide based polymers, cationic polyethyleneimines, polyamidoamines and poly(diallyldimethyl ammonium chloride) or any mixture thereof.
 27. The method according to claim 26 wherein said cationic starch is selected from the group consisting of starch tertiary aminoalkyl ether derivatives, starch quaternary ammonium ether derivatives, aminoethylated starches, starch cyanamide derivatives, starch anthranilates and cationic dialdehyde starch.
 28. The method according to claim 13 wherein at least one ionic polymer is introduced into the fiber-containing suspension containing cellulose fibers.
 29. The method according to claim 13 wherein the calcium carbonate is a precipitated calcium carbonate and wherein the calcium carbonate is calcite.
 30. The method according to claim 13 wherein the content of ultrafine particles of calcium carbonate in the calcium carbonate suspension is higher than or equal to 1% by weight and lower than or equal to 70% by weight.
 31. The method according to claim 13 wherein the ratio of the cationic polymer with respect to the calcium carbonate is higher than or equal to 0.1 wt/wt.
 32. The method according to claim 13 wherein the amount of calcium carbonate in the fiber-containing suspension containing cellulose fibers is higher than or equal to 0.01% and lower than or equal to 50% by weight calculated as dry calcium carbonate on the sum of dry fibers and optional fillers.
 33. The method according to claim 13 wherein the ionic polymer is a cationic polymer selected from the group consisting of cationic starch, cationic guar gum, cationic acrylamide based polymers, cationic polyethyleneimines, polyamidoamines and poly(diallyldimethyl ammonium chloride) or any mixture thereof.
 34. The method according to claim 33 wherein said cationic starch is selected from the group consisting of starch tertiary aminoalkyl ether derivatives, starch quaternary ammonium ether derivatives, aminoethylated starches, starch cyanamide derivatives, starch anthranilates and cationic dialdehyde starch. 